Background Selection pressure due to parasitic infestation puts adverse effects on birds’ fitness concerning survival and reproductive success. The present study was conducted on Eurasian tree sparrow at Shillong (25°34′ N, 91°53′ E), India, for a year to examine ectoparasitic infestation by Myrsidea balati and Sturnidoecus ruficeps. The parameters on the annual prevalence of infestation, annual cycles of gonadosomatic index (GSI), molt, hematocrit percentage and the time spent in behavior display were recorded. Results The infestation of both lice species ran parallel to GSI in both sexes of sparrow. Further, the intensity of infestation was higher in males. The increase in number of ectoparasites was found positively correlated with the increase in GSI in the birds. However, it was negatively correlated with feathers molt. Thus, the infestation of both parasites was maximum during the reproductive phase and minimum during the period when the birds were undergoing the process of molt. Seasonal variation in hematocrit percentage shows decline during high infection in females and during molt in both the sexes. The time spent in showing behavioral traits to limit infection was the longest during high infestation. Conclusions We conclude that the tree sparrow adopts various adaptive strategies like feathers molt and longer display of behavioral traits during their breeding and/or non-breeding seasons to control parasitic load and avoid energy conflict with high energy demanding processes like reproduction and molt.

We studied the fine histological structures of pecten oculi of the Eurasian tree sparrow using various microscopy techniques. The pecten of the tree sparrow was found to be of a pleated type comprising of pleats, bridges, and base. The light microscopic study revealed further that the pleats consist of capillaries of varying sizes, blood vessels, and numerous pigmented cells that give them a black color. Histochemical studies of pecten showed a large deposition of lipid droplets, which were more abundant in the basal area. The transmission electron microscopy displayed capillaries and blood vessels that remain surrounded by a thick fibrous basal membrane. They are formed of endothelial cells having a large lumen and abluminal area with microfolds. Interstitial spaces were found filled with rounded melanocytes, electron-dense pigment granules, and mitochondria. Observations under the scanning electron microscope revealed the presence of a dense vascular network of capillaries and vessels. In addition, large hyalocytes were also observed on the surface of the pleats. The above observations suggest that the histological structure of the pecten of the tree sparrow resembles those present in the pecten of other diurnal birds. However, further investigation is required to ascertain its functional role in birds.